The present invention relates to an electromagnetic actuator and a method for manufacturing an electromagnetic actuator, and to a control valve for a variable displacement compressor using an electromagnetic actuator.
A typical variable displacement compressor (hereinafter, simply referred to as a compressor) that forms a part of a refrigerant circuit in an air-conditioning system includes a control valve, or an externally controlled electromagnetic valve. The control valve includes an electromagnetic actuator 101 as shown in FIG. 8.
A cup-shaped cylinder 102 accommodates a stationary core 103 and a movable core 104. A coil 105 is arranged about the cylinder 102. When current is supplied to the coil 105, an electromagnetic force is generated between the stationary core 103 and the movable core 104. This causes the movable core 104 to slide along the inner surface of the cylinder 102. The force generated by the movable core 104 is communicated with a valve body (not shown) through a rod 106. The displacement of the valve body based on the movement of the movable core 104 adjusts the opening degree of the control valve, thus changing the displacement of a compressor.
For example, the displacement of a swash-plate type compressor is adjusted by changing the pressure in a crank chamber. The control valve adjusts the opening degree of a supply passage for supplying compressed refrigerant from a discharge chamber to the crank chamber.
Recently, typical air-conditioning system employs carbon dioxide as the refrigerant. In such a compressor, the pressure of the refrigerant is much higher than that of a compressor using chlorofluorocarbon as the refrigerant. Therefore, the withstanding pressure of the control valve needs to be improved to control the displacement of the compressor. Thus, the cylinder 102 that has a thick wall is used.
However, the cylinder 102 is made of nonmagnetic material to prevent the magnetic flux between the stationary core 103 and the movable core 104 from leaking. Therefore, if the wall of the cylinder 102 is excessively thick, the wall hinders the magnetic flux communicated between the coil 105 and the movable core 104. This reduces the electromagnetic force applied to the valve body by the electromagnetic actuator 101. To obtain a desired electromagnetic force, the coil 105 needs to be enlarged. This enlarges the electromagnetic actuator 101, thus enlarging the valve body.
To prevent the electromagnetic force output from the electromagnetic actuator 101 from decreasing, a part of the cylinder 102 in the vicinity of the movable core 104 may be formed of magnetic material. However, the cylinder formed of magnetic material, such as iron, generally slides less smoothly on other members formed of magnetic material compared with a cylinder formed of nonmagnetic material, such as nonmagnetic stainless steel. Therefore, the inner diameter of the magnetic part of the cylinder 102 needs to be enlarged so that the magnetic part does not contact the movable core 104. In this case, the movable core 104 is only guided by a narrow range of a nonmagnetic part of the cylinder 102. This increases the play of the movable core 104, which increases the sliding resistance between the cylinder 102 and the movable core 104. As a result, the probability that hysteresis is generated in the adjusting characteristics of the opening degree of the control valve is increased.